This invention relates generally to cryogenic coolers and, more particularly, to methods for fabricating wire screen disks for cryogenic cooler regenerators.
Regenerative cryogenic coolers, such as Stirling and pulsed tube cryogenic coolers, have been developed for cooling space-based infrared detectors to very low temperatures to provide greatly improved infrared detection sensitivities as well as a variety of ground based applications including cryopumps for high vacuum systems. Stirling and pulse tube cryogenic coolers are closed-cycle expansion coolers which produce cooling through an alternating compression and expansion of a gas, such as helium or hydrogen, with a consequent reduction of gas temperature. For example, a typical pulse tube cryogenic cooler utilizes a compressor to generate a continuous pressure wave which produces an alternating mass flow through the pulse tube cooler. The alternating pressure and mass flow is a pressure/volume work which causes a regenerator to pump heat from a cooling load through a cold end heat exchanger to an aftercooler, where the heat is rejected to a heat sink. Meanwhile, the pressure/volume work travels down the pulse tube, where it is also rejected as heat to the heat sink by a hot end heat exchanger.
The efficiency of a cryogenic cooler, which is largely determined by the efficiency of the regenerator in regenerative type cryogenic coolers, is particularly important in space applications. The regenerator is typically a stack of a thousand or more wire screen disks which act as a thermal sponge, alternately absorbing heat from the gas and then rejecting the absorbed heat to the gas as the pressure wave oscillates back and forth. For good thermal efficiency, the heat transfer between the regenerator and the gas must occur with minimum energy loss. Also, the regenerator must have a large heat capacity compared with that of the gas, as well as have low thermal conductivity along its length to minimize conduction loss. To achieve good thermal efficiency and prevent blow by, as well as provide low thermal conductivity along its length, the wire screen disks must fit properly in the regenerator, preferably with a slip fit.
Wire screen disks are typically fabricated with mechanical punches which are hand or machine driven. However, mechanical punches wear easily and must be frequently sharpened and calibrated. Also, mechanical punches cause frayed edges which prevent the disks from fitting properly in the regenerator. In addition, these frayed edges have loose and bent wires which can break off, potentially causing severe damage to the compressor and other moving parts of the cryogenic cooler. Furthermore, it is difficult to fabricate wire screen disks having complex varying geometries with mechanical punches. Accordingly, there has been a need for an improved method for fabricating wire screen disks for cryogenic cooler regenerators. The present invention clearly fulfills this need.